The speed of a hydraulically driven working member on a machine depends upon the cross-sectional area of principal narrowed orifices of the hydraulic system and the pressure drop across those orifices. To facilitate control, pressure compensating hydraulic control systems have been designed to maintain an approximately constant pressure drop across those orifices. These previous control systems include sense lines which transmit the pressure at the valve workports to a control input of a variable displacement hydraulic pump which supplies pressurized hydraulic fluid in the system. Often the greatest of the workport pressures for several working members is selected to apply to the pump control input. The resulting self-adjustment of the pump output provides an approximately constant pressure drop across each control orifice whose cross-sectional area can be controlled by the machine operator. This facilitates control because, with the pressure drop held constant, the speed of movement of each working member is determined only by the cross-sectional area of the corresponding orifice. Hydraulic systems of this type are disclosed in U.S. Pat. Nos. 4,693,272 and 5,579,642, the disclosures in which are incorporated herein by reference.
With this type of system, all of the loads receive the same supply pressure. When the maximum flow capacity of the pump is reached, the supply of fluid to all actuators is diminished. However, when the maximum pump capacity is reached in some applications, it is desirable to maintain as great a flow as possible to certain actuators, even at the expense of a greater flow reduction to the other actuators. For example, in an industrial truck, the pump supplies a load lifting mechanism and hydraulic motors which drive the wheels. If the operator attempts to raise a heavy load while the truck is moving forward, the maximum pump flow capacity may be reached causing the forward movement to slow. In this situation, it is preferable to maintain the forward speed and raise the load at whatever rate can be achieved without affecting forward movement of the industrial truck.